It has long been recognized that coalbeds often contain combustible gaseous hydrocarbons that are trapped within the coal seam. Methane, the major combustible component of natural gas, accounts for roughly 95% of these gaseous hydrocarbons. Coal beds may also contain smaller amounts of higher molecular weight gaseous hydrocarbons, such as ethane and propane. These gases attach to the porous surface of the coal at the molecular level, and are held in place by the hydrostatic pressure exerted by groundwater surrounding the coal bed.
The methane trapped in a coalbed seam will desorb when the pressure on the coalbed is sufficiently reduced. This occurs, for example, when the groundwater in the area is removed either by mining or drilling. The release of methane during coal mining is a well-known danger in the coal extraction process. Methane is highly flammable and may explode in the presence of a spark or flame. For this reason, much effort has been expended in the past to vent this gas away as a part of a coal mining operation.
In more recent times, the technology has been developed to recover the methane trapped in coalbeds for use as natural gas fuel. The world's total, extractable coal-bed methane (CBM) reserve is estimated to be about 400 trillion cubic feet. Much of this CBM is trapped in coal beds that are too deep to mine for coal, but are easily reachable with wells using drilling techniques developed for conventional oil and natural gas extraction. Recent spikes in the spot price of natural gas, combined with the positive environmental aspects of the use of natural gas as a fuel source, has hastened development of coal-bed method recovery methods.
The first research in CBM extraction was performed in the 1970's, exploring the feasibility of recovering methane from coal beds in the Black Warrior Basin of northeast Alabama. CBM has been commercially extracted in the Arkoma Basin (comprising western Arkansas and eastern Oklahoma) since 1988. As of March 2000, the Arkoma Basin contained 377 producing CBM wells, with an average yield of 80,000 cubic feet of methane per day. Today, CBM accounts for about 7% of the total production of natural gas in the United States.
While some aspects of CBM extraction are common to the more traditional means of extracting oil, natural gas, and other hydrocarbon fuels, some of the problems faced in CBM extraction are unique. One common method generally used to extract hydrocarbon fuels from within minerals is hydraulic fracturing. Using this technique, a fracturing fluid is sent down a well under sufficient pressure to fracture the face of the mineral formation at the end of the well. Fracturing releases the hydrocarbon trapped within, and the hydrocarbon may then be extracted through the well. A proppant, such as course sand or sintered bauxite, is often added to the fracturing fluid to increase its effectiveness. As the pressure on the face of the fractured mineral is released to allow for the extraction of the hydrocarbon fuel, the fracture in the formation would normally close back up. When proppants are added to the fracturing fluid, however, the fracture does not close completely because it is held open by the proppant material. A channel is thus formed through which the trapped hydrocarbons may escape after pressure is released.
Although course fracturing of this type is very successful in some applications, it has not proven particularly useful in the recovery of CBM. Coal fines recovered with the water and methane during CBM extraction will quickly foul the well when course fracturing techniques are used. This necessitates the frequent stoppage of CBM recovery in order that the production tubing may be swabbed or cleaned. It has been found that course fracturing will significantly reduce both the long-term productivity and ultimate useful life of a CBM well.
While traditional fracturing has proven unsuccessful in CBM extraction, all coal beds contain cleats, that is, natural fractures through which CBM may escape. As hydrostatic pressure is decreased at the cleat by the removal of groundwater, methane within the coal will naturally desorb and move into the cleat system, where it may flow out of the coal bed. CBM may thus be withdrawn from the coalbed in this manner through the well, without the necessity in many cases of any artificial fracturing methods. CBM exploration and well placement strategies thus are highly dependent upon a good knowledge of cleat placement within the coalbed of interest.
If artificial fracturing processes are used to stimulate production in CBM wells, they must be very gentle so as not to harm the coalbed cleats, and thereby reduce rather than increase well production. Acids, xylene-toluene, gasoline-benzene-diesel, condensate-strong solvents, bleaches, and course-grain sand have been found to be detrimental to good cleat maintenance. Recent experience in coalbeds in the Arkoma Basin indicates that a mixture of fresh water with a biocide, combined with a minimal amount of friction reducer, may be the least damaging fracturing fluid. The failure to use gentle fracturing methods and other good production practices elsewhere in a coal bed can even damage production at nearby wells.
Regardless of whether a fracturing liquid is used in CBM extraction, some means must be provided for the removal of the significant quantity of groundwater expelled as a result of the process. One study found that the average CBM well removed about 12,000 gallons of water per day. Pump jacks and surfactant (soap) introduction are the most common means of removing this water. Pump jacks, which have been used for decades in traditional petroleum extraction, simply pump water out of the well by mechanical means. A pump is placed downhole, and is connected to a rocking-beam activator at the wellhead by means of an interconnected series of rods. Pump jacks are expensive to install, operate, and maintain, particularly in CBM applications where bore cleaning is required more often due to the presence of coal fines. The presence of the pump jack at the end of the well also requires lengthier downtimes when maintenance is performed, reducing the cost-efficiency of the well.
In contrast to the pump jack method, the surfactant method relies upon the hydrostatic pressure within the well itself to force groundwater up through the borehole and out of the extraction area. The surfactant combines with the groundwater to form a foam, which is pushed back up through the well by hydrostatic pressure. The water/surfactant mixture is then separated from the devolved methane gas and disposed of by appropriate means. Ideally, not all water is removed at the point of CBM extraction; rather, only enough water is removed such that the hydrostatic pressure in the area of the borehole is reduced just enough that the methane bound to the coal will desorb. In this way, damage to the coalbed cleats in the area of the borehole is minimized. Care must be exercised to prevent the surfactant from entering the coal formation, since this too may damage the coalbed cleats and reduce the production rate and lifetime of the well.
Two methods are commonly used today for the introduction of surfactant into a CBM well. One method is the dropping of “soap sticks” into the well. The soap sticks form a foam as they are contacted by water rising up through the well, thereby forming foam that travels up and out of the well due to hydrostatic pressure. The second method is to attach a small tube inside the main production tube and pour gelled surfactant into this tube. The surfactant travels down the tube through the force of gravity, capillary action, or its own head pressure, eventually depositing the gel into the flow of water in the well and forming a foam. Again, this foam rises back up through the well for eventual removal. Use of either of these methods is believed by the inventor to increase well production on average by 10-20%.
Although a significant amount of CBM is extracted through vertical drilling methods, horizontal drilling methods have become more common. The general techniques for horizontal drilling are well known, and were developed for conventional extraction of oil and natural gas. In the usual case, the well begins into the ground vertically, then arcs through some degree of curvature to travel in a generally horizontal direction. Horizontal wells thus contain a bend or “elbow,” the severity of which is determined by the drilling technique used. It is believed that horizontal drilling may result in better extraction rates of CBM from many coal beds due to the way in which coalbeds tend to form in long, horizontal strata. One analysis has shown that “face” cleats in coalbeds appear to be more than five times as permeable as “butt” cleats, which form orthogonally to face cleats. A horizontal well can increase productivity by orienting the lateral section of the well across the higher-permeability face cleats. As a result of these effects, the area drained by a horizontal well may be effectively much larger than the area drained by a corresponding vertical well placed into the same coalbed stratum. Horizontal well CBM extraction thus promises greater production from fewer wells in a given coalbed. The first horizontally drilled CBM wells in the Arkoma Basin were put in place around 1998.
While horizontal drilling promises improved theoretical productivity over vertical drilling in many instances, it raises several problems of its own that are unique to CBM extraction. It may be seen that the deposit of a “soap stick” in a horizontal well will result in the movement of the soap stick only to the bottom of the “elbow” of the well. The soap stick is carried by gravity to this point, but will not proceed past the point where the well turns. Thus this method will form no foam at the end of the well bore at all; foam is only formed at the point where the soap stick comes to rest. The inventor has recognized that increased productivity would result from the production of foam at the end of the well, which is just at the point where the water is being extracted from the coal bed seam. The soap stick will never reach this point.
Likewise, the method of introducing a surfactant by dripping a gel into the well also suffers when horizontal drilling techniques are used. Gravity, capillary action, or head pressure are the only agents moving the gel down into the well. In actual practice, the lines used to deliver this gel (typically ⅜ inch stainless steel tubing) cannot be made to reach to the bottom of the well, since the weight of the capillary tubing is not sufficient to overcome the frictional force arising from contact with the tubing walls, due to the arc in the horizontal well “elbow.” Again, as in the case of the soap stick, foam will not be formed at the end of the well where it is needed most.
Another disadvantage of the gel capillary tube approach is that the tubing is employed inside the main production tube in the well; thus when the main production tube plugs or otherwise requires maintenance, the gel delivery tubing will impede efforts to clean, clear, or otherwise maintain the production tube. This is a particular problem in CBM extraction because of the fouling problems presented by coal fines, and the resulting need to regularly swab or clean the well tubing. Finally, since the gel is not introduced under pressure, it cannot adjust to the hydrostatic pressure at the end of the well. This pressure is dependent upon the depth of the well and the height of the water table. If the hydrostatic pressure is significantly less than the gel pressure, then the gel may flow out the production tube and into the coal bed, thereby damaging the coal bed cleats and retarding future production. If the hydrostatic pressure is significantly greater than the gel pressure, then the gel will flow little or not at all, producing minimal foam and impeding removal of groundwater and thus reducing CBM extraction rates.
While this discussion has focused on CBM extraction, another developing area for the recovery of natural gas from unconventional sources is the extraction of natural gas from sandstone deposits. Sandstone formations with less than 0.1 millidarcy permeability, known as “tight gas sands,” are known to contain significant volumes of natural gas. The United States holds a huge quantity of these sandstones. Some estimates place the total gas-in-place in the United States in tight gas stands to be around 15 quadrillion cubic feet. Only a small portion of this gas is, however, recoverable with existing technology. Annual production in the United States today is about two to three trillion cubic feet. Many of the same problems presented in CBM extraction are also faced by those attempting to recover natural gas from tight gas sands, and thus efforts to overcome problems in CBM extraction may be directly applicable to recovery from tight gas sands as well.
What is desired then is an apparatus for and method of introducing surfactant into a borehole for CBM extraction, tight sand gas extraction, or other types of gas-recovery options, where such apparatus and method is well-suited to horizontally drilled wells and that produces foam at the tip of the borehole for optimal groundwater removal, while preventing the flow of surfactant into the formation itself in conditions of potentially varying hydrostatic pressure.